This invention relates generally to liquid chromatography, and more specifically relates to a solvent supply system for use in high performance column liquid chromatography.
Chromatography is a separation method wherein a mixture of components (called the "sample" or "sample mixture") is placed as a zone at one end of a system containing a stationary phase and a mobile phase. Each component of the sample distributes itself in dynamic equilibrium between the two phases in a ratio characteristic of that component. As a result, the flowing mobile phase causes each individual component zone to migrate at a characteristic rate, and the zones become separated after a period of time.
There are various types of chromatography, e.g., liquid chromatography, gas chromotography, thin-layer chromatography, etc. The major differences between these various chromatographic methods lies in the physical state of the mobile phases (gas or liquid), and the manner in which the stationary phase is supported, e.g., coated on an inert granular material packed in a tube, coated on an inner wall surface, etc. In all these methods, the separation mechanism is essentially the same, i.e., distribution of the sample components between a mobile phase and a stationary phase. When the method is used for chemical analysis, a detector is commonly placed at the far end of the system to monitor the passage of the component zones as they emerge from the system. The signal from the detector is displayed on a recording device such as a strip chart recorder, and the record indicates both qualitative and quantitative information regarding the components of the sample.
It is often desirable for a chromatographic system to provide high resolution (i.e., a large degree of component separation with narrow zones), evenly spaced component zones, rapid separation, and a satisfactory record from a very small sample. The behavior of the system described in these terms may be called the "performance" of the system. It is well known in the chromatography art to improve system performance by changing one of the following system variables during the course of the analysis: temperature, chemical composition of the mobile phase, and flow rate of the mobile phase. For example, in gas chromatography the temperature of the system is often varied as a preselected function of time. This technique is known as "temperature programming", and it improves the performance of the system, especially with samples containing components which boil over a wide temperature range. Analogous to temperature programming in gas chromatography is the use of "gradient elution" in liquid chromatography. Gradient elution refers to changing the chemical composition of the mobile phase (also called the "eluent" or "eluting solvent") as a function of time, thereby improving the performance of the system, especially with samples containing components which vary widely in chemical properties. The net effect of gradient elution is to shorten the retention time of compounds strongly retained on the columns without sacrifice in separation of early eluting compounds. Further details regarding the fundamentals of gradient elution techniques may be found in numerous sources in the prior art, as, for example, in the publication by L. R. Snyder appearing in Chromatograhy Review 7, 1 (1965).
A central concern pertinent to liquid chromatography apparatus of the type considered herein, is one of providing a proper flow of solvent to and through the chromatographic column. Thus in the past, numerous and varied approaches have been utilized for supplying solvents to high performance liquid chromatographic columns. A key requirement in this connection is one of providing a relatively non-pulsating, (i.e., a constant) flow of solvent -- in that the LC detector is sensitive to flow variations, and can provide erroneous readings and exhibit excessive noise in the presence of pulsating flow. Various approaches have been utilized in the past in order to enable such result; but in general, the prior art methodology directed at such end has involved highly expensive and overly complex mechanisms. Thus, in a typical example wherein a system is intended for operation in a gradient elution mode, i.e., by use of two distinct solvents, a dual cylinder pump arrangement has been utilized. Such an arrangement requires two distinct cylinder pumps, including separate means for driving each of the pumps, thus requiring separate speed controls, etc.
In principle, it would seem that many of the above-mentioned problems arising in connection with the solvent pumping systems of the prior art, might be overcome by use of a single cylinder arrangement in cooperation with a relatively small displacement volume reciprocating piston. A principal deterrent to the use of this arrangement, however, has been the fact that the ensuing flow will, by its nature, be pulsating -- particularly at low flow rates. Further, the nature of the pulses present in the flow is such that they are not easily removed by filtering; and the presence of such pulses can sharply limit the efficiency of the detector system. It is understood in the foregoing connection that the word "piston" as used in the specification includes pistons where the seal remains fixed relative to the moving member and plungers where the seal is fixed with respect to the stationary cylinder.
It has in the foregoing connection been long recognized that the aspect of the reciprocating pump which is principally responsible for an unacceptable pulsating flow is the fact that, when the pump piston is driven by a simple crank shaft mechanism, the axial displacement of the piston as a function of time is sinusoidal. This implies, of course, the presence of equal time-spaced pressure (or liquid pumping) pulses, alternating with fill periods of duration equal to the pressure pulse duration for the pump chamber. In an effort to overcome this pattern, it has been proposed to drive the piston by suitably shaped cams. Pursuant to such approach, these cams serve to alter the time displacement function of the pump piston so as to foreshorten the fill portion of the cycle in comparison to the pumping portion of the cycle, and in some instances to render the movement during pumping relatively linear in nature, i.e., to render the displacement linear as a function of time. This sort of arrangement does have the advantage of changing the form of the pulsating pattern so as to diminish the pulsing and render filtering of the remaining pulses more feasible. However, this approach is less than satisfactory in a most important respect. In particular, the cam represents a fixed pattern, and thus provides a fixed relationship or ratio between the fill and pumping portions of the pump cycle. And yet, in many instances it is desired to have a capability for operation over various flow rates -- which indeed can vary very widely. If, however, the flow rate is increased by merely increasing the rate of cam rotation, then the fill portion of the cycle becomes successively shortened -- and can reach a point where insufficient feed time is available, leading to cavitation and other problems.
In a U.S. Pat. No. 3,985,021 granted to the present inventors, and entitled HIGH PERFORMANCE LIQUID CHROMATOGRAPHY SYSTEM, which patent is assigned to the same assignee as is the present application, there is disclosed a liquid chromatography system which is particularly useful in overcoming the aforementioned flow problems. The system described in that patent includes a reservoir for a liquid mobile phase, a liquid chromatographic column, reciprocating pumping means for pumping the mobile phase through the column, and motor means for driving the pumping means through successive reciprocation cycles. Means are provided further, for controlling the rotational speed of the motor throughout the reciprocation cycle of the pump so as to provide preselected average rotational speeds over predetermined subintervals of each successive reciprocation cycle. Application of the control cycle is synchronized with the pumping cycle so that the speed control is properly applied over each successive reciprocation cycle.
A further problem evidenced in the prior art, including in the systems of the type just considered, arises in connection with the inlet valve commonly utilized to control flow proceeding toward the pump chamber. In the past it has been common to use a simple check valve for such purpose, the valve usually being of the gravity-biased type. In practice, however, much difficulty has been experienced with such devices. In particular, the pressure difference across the check valve is so slight as to make operation unreliable -- even minor amounts of particulate matter can severly impair operation of the valve. Further, the reliance on gravity to close the check valve creates another source of serious difficulties.
In accordance with the foregoing, it is an object of the present invention, to provide a high performance, high pressure liquid chromatography system incorporating an inlet valve structure for controlling flow to the system pump, wherein valve actuation is positive, thereby enabling high reliability in the operation of such valve, and completely accurate control of the opening and closing points thereof.